Sleep literally cleans your brain. During slumber, more cerebrospinal fluid flushes through the brain to wash away harmful proteins and toxins that build up during the day.
Harmful build up of proteins and brain toxins can lead to neurological damage. Many dementia patients have a difficult time sleeping. They can never "switch off" their brains in order to rest. The brain is in constant "on" mode which can lead to hallucinations, temper and mood changes.
Throughout the series, the castaways were shown constantly on the move, day and night, mission after mission, worn down by lack of sleep. The physical strains of island survival took a mental toll on them. They became irritable, possessive, paranoid, abusive and sly. Even level headed Sawyer showed those various traits as the days and weeks passed on the island.
If the first theme of the show was the standard "how would you survive on a deserted island," then the basic survival instincts would take charge of your body. The gut instinct of fear of the unknown would be front and center in your mind. What is behind the bushes? What is that noise? Is something out there that can harm me?
That is why the castaways felt compelled to stay together; strength in numbers. That is also why they chose the beach to set up camp; they only had to worry about the land side at night.
Getting past the fear, castaways in this situation would have four things on their mind: food, water, shelter and rescue. The island seemed to have sufficient plant life to provide some basic nutrition. Water was the first problem that needed to be solved which led to exploring the island. Shelter was from the airplane debris which kept the castaways focused on something else besides their plight.
The one issue that did not take center stage was rescue. It was more an afterthought than a compelling mission. Even when they found a way by finding the cockpit radio, things stopped by a tragic death. Only when the Others created a more dramatic need for survival did the main characters, as leaders, tried to find a way off the island. Michael's boat was really the first and last chance. When the freighter arrived, a second set of danger emerged which left most of the castaways unable to escape.
Throughout the incidents, it seemed that main characters stopped thinking rationally - - - asking the key questions to their colleagues. Information was sparsely communicated on a need to know basis. This led to jealousy and splits among the group. The island began to assert a deranged assertiveness in both Jack and Locke which drove a stake between a combined effort to leave the island.
At one point, Hurley hallucinations became so real that he almost killed himself by jumping off a cliff. His friend, who may have been imaginary, almost got him to buy into the premise that the only way to leave the island was to die. In some respects, this was a true statement. (Anti-purgatory theorists will not fixate on the Ending church as anything particular to island life.)
Hurley was the world in which the other characters orbited. Hurley was the only character to truly fit into all the castaway sub-groups and with the Others. (He was let go without any torture or retribution.) Some theorists believed that the entire show was within Hurley's own mind. A sleep depraved mind that got the story line farther and farther away from reality as each season ended. Hurley was a known mental patient - - - who seemed to get along with all the day room patients just like he did with the island people. He was not special. He was not a forceful personality. He was not a danger. He was the perfect observer.
Or, in the analogy to another fantasy, he could have been the Wizard behind Oz's curtain.
Collective dream theorists think that Hurley could have been the "thought engine" that connected the various characters subconscious dreams, desires, thoughts and issues to "life" on an imaginary island world. Dreams and a weakened mental state was suggested as the reason why the story lines had so many continuity errors and dead ends.
With so many tangents weaved into the LOST episodes, it is not difficult losing sleep over trying to figure everything out.